Power Factor Correction FAQ

[jonnyGURU]

What is "power factor"?

Power factor, or ?PF? for short, is the ratio of the real power to the apparent power.

Real power is the capacity of the circuit for performing work in a particular time and is measured in Watts.

Apparent power is the product of the voltage and current (V x A) of the circuit and is measured in volt-amperage (or ?VA?.)

I know that it almost sounds as if Watts and VA are the same thing, and in DC they are (240W DC is equal to 240VA DC, for example) but because energy stored in the load of a device using alternating current (AC) is returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can actually be greater than the real power. This would give you a power factor of less than 1. Power factor below .70 is generally considered poor power factor.

So what if I have a poor power factor?

Most consumers are charged per kWh (kilowatt hour) by their utility company. Fortunately, for the customer, poor power factor does not typically affect how much wattage your computer uses. But poor power factor does has an affect on how much power the utility companies can deliver. This means that the utility companies either have to increase their grid's capacity to compensate for the increased power load, charge per kVA instead of per kWh (some commercial/industrial accounts are charged per kVA while residential customers are still charged per kWh), charge a "power factor penalty charge" (which can be applied to customers with power factors even as high as .95!) or impose a martial law of sorts requiring all appliances sold in the country to have a power factor of .96 or better.

The European Union believed the latter of these to be the best solution, so as of January 1st of 2001 the EN61000-3-2 was put into place imposing limits on the harmonic currents drawn from the mains. In other words, if you're in the EU, you are REQUIRED to have a power supply with power factor correction. Power factor correction is not (yet) a requirement in the U.S.

Poor power factor can also limit how much current you can draw from a circuit. If you?re using a 20A breaker and are drawing a total of 15A in ?real power? and the power factor is only .70, then you are drawing an apparent 21.4A, thus overloading the breaker.

Also, one of the requirements for a computer power supply to be considered "Energy Star" compliant is that it has a power factor of at least .90.

What is power factor correction?

Poor power factor can be corrected by adding some form of power factor correction to the AC input of the power supply. Power Factor Correction comes in two forms: Active Power Factor Correction, or APFC, and Passive Power Factor Correction.

Computer power supplies can create harmonics of the same frequency as the input current, due to the non-linear load caused by the bridge-rectifier doing the AC to DC conversion, and typically have poor power factor (typically 0.55 to 0.65).

Passive Power Factor Correction uses a filter that kills any harmonic current and passes current only at line frequency (typically 60Hz in the U.S.) The filters typically come in the form of large, high-value inductors.

Active Power Factor Correction is done by using a boost converter in between the bridge-rectifier and main input capacitors. The boost converter attempts to maintain a constant output voltage while drawing a current that is always in phase and at the same frequency as the line voltage.

Power factor correction won?t make your power supply more efficient (convert more DC output power with less AC input power), but can allow for more devices to be plugged into the same circuit. If you have a number of PCs on the same circuit, say in the event of a LAN party where a number of computers are plugged into a single power strip, it is easier to overload that circuit if a number of the PCs have poor power factor. Say for example you have a 20A breaker and there are five PCs plugged into the outlets on this breaker. Let?s assume the PCs are each drawing 115V at 3A from the wall, or 345W each, for a total of 1725W. This isn?t a lot of power and something the breaker should be able to handle without problem, but if the computers in question lack power factor correction, the ?apparent? current draw could be as high as 27A (assuming a power factor of .55)! This will easily trip the breaker.

So I'm in the U.S. PFC is a non-issue, right?

Well... yes and no.

You may not NEED power factor correction, but "green" is more marketable and it costs a PSU factory less to make a bunch of the same platform, even if it includes PFC, then split production up between non-PFC and PFC designs.

These days, PFC is typically integrated into the design of the platform. So much so that the PSU manufacturers couldn't even really REMOVE the PFC in an effort to cut costs. That's ok because they're making up for it in larger quantities being able to sell their product around the globe. It also reduces returns because with active PFC it's impossible to plug the PSU into 230V while the switch is set to 115V (BOOM!)

If you're looking at a PSU over 550W continuous and it DOES NOT have PFC, be suspicious of it. It's likely an old, recycled platform that's inefficient and probably not suitable for the high +12V loads of today's computers. Or worse: It's really a 500W PSU labeled as a 700W! :Q

 

[legocitytruck]

With a PSU with PFC be necessarily a more reliable PSU?

 

[HOOfan 1]

Originally posted by: legocitytruck
With a PSU with PFC be necessarily a more reliable PSU?

Will the most reliable PSUs have APFC? Yes because they are expensive enough that it would be stupid for them not to include APFC.

Does APFC make a PSU more reliable. No not at all. There are some PSUs with no APFC that are more reliable with some that do have APFC.

 

[theAnimal]

Bump for re-sticky.

 

[LiuKangBakinPie]

Pfc is just to sell it in europe.

 

[SuperMarioBro]

I gotta be honest, there's a few things I really didn't understand at all here. First of all, how is the electric company even able to know the "real power" that a consumer uses? Wouldn't they only be aware of the apparent power? Also, how is it possible to increase the PFC and NOT increase the efficiency of the device? If the ratio of real power to apparent power increases, isn't that kinda the same thing as increasing the efficiency?

Sounds like an interesting topic, though. So if anyone knows of a PFC-for-dummies kinda thing I could read up on, I'd love to take a look.

 

[mindless1]

^ My electric meter and all I've ever seen (I don't go around checking electric meters while traveling the US) in residential use do only measure real power, not apparent power.

As for efficiency think of it like this: You can transport cups of water from one place or the other and get the same job done by carrying 2 cups at a time, OR carrying 1 cup then alternating to carrying 3 cups every other time. The difference is you need an additional cup to do the latter. Same thing with power, the power company and your breakers, wiring, etc, has to have higher capacity for the apparent power because it's not as steady (when broken down into sub-second periods) but with apparent power there are periods where the current is less than the real power average... like if you were carrying 1 cup of water instead of 2.

 

[SuperMarioBro]

^^^
Thanks, that certainly makes some sense. Someone sent me a video I'll need to check out soon, too.

 

[Smoblikat]

Makes snese.

 

[Mark R]

I've just become aware of a new type of active PFC circuit that is slowly coming into use, should provide further efficiency benefits over and above that of 1st generation aPFC.

First generation aPFC, takes DC power, just after it has been rectified from AC, and then boosts the voltage, so that the main PSU receives 385-390V DC. This voltage boost (from 320-340 VDC) significantly improves the energy efficiency of the main PSU, which is a useful side effect. However, the aPFC circuit does consume energy, so the total energy consumed in the PSU is little changed (and may even be increased marginally).

2nd generation aPFC is used instead of rectifier diodes. Rectifier diodes are relatively lossy, and current needs to flow through 2 of 4 rectifier diodes, to be converted to DC, and then a 3rd rectifier (extra-lossy, because it is under very high stress) in the 1st gen aPFC circuit. In 2nd gen aPFC, the 2 of the rectifier diodes are replaced with high-efficiency MOSFETs. This permits power-factor control (and voltage boost), while simultaneously replacing high-loss diodes with low-loss MOSFETS, and ditching the extra MOSFET/diode of 1st gen aPFC.

 

[pitz]

In an office environment, where a large number of computers are fed off of what might be a transformer from a higher voltage (ie: a 480/277 feed is relatively common, with step-down transformers to 120/208) -- power factor correction style PSU's, which ostensibly consume power with less total harmonic distortion, will reduce the loading/heating on the transformer.

Older step-down transformers were designed with minimallly sized neutral circuits in the 3-phase* neutral, on the assumption that currents would be balanced and cancel each other out in the 3phase transformer. The 'math' changes completely when you have 3rd and higher order harmonic components of the electricity consumption, and far larger neutral circuits are typically required. Many a'buildings' transformers were destroyed or overheated when the occupants failed to realize this reality and upgrade to more suitable systems.

"Power factor", as it relates to non-linear electronics, isn't simply a phase lead/lag issue as it might be with a tradtional linear device like a motor, capacitive device, etc., but generally relates to harmonic distortion and the non-linear, non-sinusoidal absorption of real power relative to apparent power. For this reason, short of sophisticated devices such as static var compensators (solid state devices that can add or subtract real power that are controlled in real-time by microprocessors) being placed on the power system, the only viable way of dealing with the issue is through PSU redesign to reduce (but not eliminate) the problems.

Of course, it is impossible to turn a non-linear load into a purely linear load. But at least with these PFC-style power supplies, they don't look as ugly and non-sinusoidal, from a current waveform point of view, as the supplies that simply had a diode bridge on the front end.

* 3phase is the style of electrical distribution used worldwide in power systems, where, if a reference phase is defined as 120sqrt(2)cos (60*2pi*t), then the other two phases are defined as 120sqrt(2)cos(60*2pi*t + (2/3)pi) and 120sqrt(2)cos(60*2pi*t+ (4/3)pi), or in other words, the phases are spaced 2/3pi radians apart (120 degrees). It has nothing to do with the motherboard makers running around calling their embedded multiple DC power supplies 'phases' as you might see with so-called "6-phase" motherboards.

 

[pitz]

As for efficiency think of it like this: You can transport cups of water from one place or the other and get the same job done by carrying 2 cups at a time, OR carrying 1 cup then alternating to carrying 3 cups every other time. The difference is you need an additional cup to do the latter. Same thing with power, the power company and your breakers, wiring, etc, has to have higher capacity for the apparent power because it's not as steady (when broken down into sub-second periods) but with apparent power there are periods where the current is less than the real power average... like if you were carrying 1 cup of water instead of 2.

Here's a way to think of it. Imagine you connect an ideal capacitor to an ideal inductor. Neither of them have any resistance, they are ideal.

And then you somehow give them both a jolt of energy. Because they're connected together, they will pass that energy back and forth, at a rate of (w = 1/sqrt(LC)), ie: the resonant frequency.

In theory, if there is no resistance whatsoever, the currents that are induced will become extremely large. But since there is no resistance, there will be no energy dissipation. In theory, this can continue forever, as a superconducting resonator.

Practical circuits have resistance. But as energy is passed back and forth between inductances and capacitances, no actual energy is dissipated usefully in the inductance or capacitance, but the lines between the capacitances and the inductances have to be sized large enough so they can handle the currents and voltages induced.

This is where power factor comes in, it is a measure of the amount of real power dissipated (ie: heat or work in a load), as a ratio to the amount of apparent power (ie: those currents) that are induced thereto.

Basically, in a system with low power factor, there is a lot of heat lost in when energy moves between being stored as charge in terms of an electric field in a capacitor, and in terms of energy being stored in inductances as kinetic energy.

The same phenomena occurs in mechanical systems as well. Potential energy versus kinetic energy.

 

[GeeKayCee]

To make a long FAQ short, to the masses of regular PC users out there, APFC doesn't mean anything, at the end. Most of them will never need or notice its presence.

Aside for compatibily with UPS's, any manufacturer claims that go like "APFC helps your computer" are just marketing nonsense.

 

[William Gaatjes]

I've just become aware of a new type of active PFC circuit that is slowly coming into use, should provide further efficiency benefits over and above that of 1st generation aPFC.

First generation aPFC, takes DC power, just after it has been rectified from AC, and then boosts the voltage, so that the main PSU receives 385-390V DC. This voltage boost (from 320-340 VDC) significantly improves the energy efficiency of the main PSU, which is a useful side effect. However, the aPFC circuit does consume energy, so the total energy consumed in the PSU is little changed (and may even be increased marginally).

2nd generation aPFC is used instead of rectifier diodes. Rectifier diodes are relatively lossy, and current needs to flow through 2 of 4 rectifier diodes, to be converted to DC, and then a 3rd rectifier (extra-lossy, because it is under very high stress) in the 1st gen aPFC circuit. In 2nd gen aPFC, the 2 of the rectifier diodes are replaced with high-efficiency MOSFETs. This permits power-factor control (and voltage boost), while simultaneously replacing high-loss diodes with low-loss MOSFETS, and ditching the extra MOSFET/diode of 1st gen aPFC.

That is great news. But it will be a while before we see that. Also, high voltage mosfets have higher RDSon. But since the development of sicarbide mosfets, a whole new generation of low rdson high voltage mosfets have arrived. I Played with carbide mosfets of 0.08 Ohm while having a drain source max voltage of 1200 Volts. The only issue is the asymmetric gate drive circuits needed. And these mosfets are still very expensive at the moment because of being new. Yet in time i would not be surprised that these carbidmosfets replace traditional mosfets. And Infineon has a whole new family of low RDSon and high DS max traditional mosfets.

http://en.wikipedia.org/wiki/Silicon_carbide#Power_electronic_devices

 

[Abwx]

The only advantage of PFC is for the electricity supplier...

Without PFC it s the supplier that pay for the reactive energy compensation, with a PFC the device store the required energy to do the compensation and dump it when needed, as such the reactive power is accounted in the consumer bill, that s why industries are required to have no less than 0.95 power factor in europe while the ratio is 0.7 for the average joe.

 

[MongGrel]

So how's Oklahoma Wolf doing over on OCF these days after the twits banned me from there ?

I used to like it there, they seem to have a hair up their ass these days.

NM, just wondered a bit.

I got kicked off there for defending some guy getting preyed upon in the classies

 

[Tr4nd]

" It's really a 500W PSU labeled as a 700W! "

Retailers nowadays...

 

[yicai12]

Sounds like an interesting topic,thank you for sharing.

 

[VirtualLarry]

Is there a list anywhere on the internet of PSUs with APFC that are also compatible with the more common stepped-sine-wave UPSes? I'm in the market for potentially two or maybe three of them.

 

[Onus]

Rather than address this at the PSU, I've been using the Cyberpower APFC UPS units on my systems.
I have a Delta-built Antec SG650 that will not run at all on stepped sinewaves, but is quite happy with the waveform from the Cyberpower (I believe it is a clipped triangle, but do not own an oscilloscope myself).
I believe it is related to "lingering" at 0V as the polarity switches, which the SG sees as bad power and shuts off. A clipped triangle crosses 0V in a continuous manner, much like a sine wave.

 

[Nec_V20]

Probably because everyone who knows thinks it is so obvious that it is not worth mentioning, however I found it strange that this tidbit was missing from the thread.

Power Supply Units are most efficient at 50% +/- 5% of their rated maximum wattage (not peak wattage which is a BS value and should never be mentioned).

What this means say for something like the Corsair AX860 is that it will convert the highest percentage of AC to DC and the least percentage will be wasted as heat in the output range between 387-473 Watts.

This is especially important if you are building a system in a small case. The less heat sources there are, the better.

 

[Bozo]

Great information

 

[jonnyGURU]

Power Supply Units are most efficient at 50% +/- 5% of their rated maximum wattage

This is largely untrue.

(not peak wattage which is a BS value and should never be mentioned).

Don't believe this was ever mentioned.

What this means say for something like the Corsair AX860 is that it will convert the highest percentage of AC to DC and the least percentage will be wasted as heat in the output range between 387-473 Watts.

You're talking about efficiency and this post is about power factor correction.